Hoist and winches are commonly used on helicopters and ships to haul, pull, raise, and lower heavy loads. Winches include a drum that supports a spool of cable that runs to the load, usually through one or more sheaves. The cable may be formed of steel or rope depending upon the use and environment of the winch. The winch spools and unspools the cable by rotating the drum. As the cable is taken in it is spooled onto the drum in multilayers. Cable or hoist/winch damage could occur as the cable is spooled if the fleet angle between the cable and the spool axis becomes too large, causing the cable to drag over adjacent wraps. Excessive fleet angle can also induce severe safety concern to the helicopter if the cable tangles with the body of the helicopter. External forces can also damage the hoist and winches if its fleet angle is too large. Such external forces could include wind, drag as the load at the end of the cable is pulled through water, or forces caused by the motor as the motor spools and unspools the cable.
Fleet angle is the angle between the center axis of alignment and the cable. Generally the center axis of alignment is defined as the axis where the cable would hang straight down if no other force other than gravity were acting upon it. By maintaining an acceptable fleet angle, the drum of the winch can spool the cable without causing the cable to drag over and wear adjacent wraps. In the past, a mechanical tracking device referred to as a follower has been used to guide the cable as the cable is spooled and unspooled. However, followers are prone to small timing errors that accumulate as the cable changes in diameter over time and use. The fleet angle of the cable may also be controlled through drum controls that vary the rotational speed of the drum, but the drum controls require sensors that are able to provide accurate, real-time measurements of the fleet angle of the cable.
In some applications, obtaining accurate, real-time measurements of the fleet angle of the cable is challenging because the cable is constantly vibrating, swaying, or bouncing. One such application is sonar dipping. In sonar dipping, a winch is mounted to a helicopter. The winch lowers an electric supporting cable with a specialized sonar for submersion in water to detect the presence of submarines. To take measurements at spaced intervals, the winch repeatedly raises and lowers the sonar at high speeds averaging about five meters per second. Because of the rapid and erratic movement of the cable, there is a need for a fleet angle sensor with high resolution and fast response time to accurately measure the fleet angle of the cable.